1. Field of the Invention
The present invention relates to an AlGaN substrate and a production method thereof.
2. Description of the Prior Art
A Group III nitride semiconductor has heretofore been utilized as a functional material for configuring Group III nitride semiconductor light-emitting devices of p-n junction structure, such as Light-Emitting Diodes (LEDs), Laser Diodes (LDs), etc., emitting visible light of short wavelength. In this case, in configuring an LED having a light-emitting layer of gallium indium nitride (GaInN), for example, to enhance the quality of the light-emitting layer and emitting light of blue or green band, a layer of gallium nitride (GaN) has been formed on a substrate in a thickness of several μm (hereinafter called an “under layer”) to improve the crystallinity thereof and make it easy to extract light. When fabricating a device necessitating crystallinity of higher quality, such as an LD, a crystal has been deposited on a processed substrate or a processed under layer in order to enhance the crystallinity of the under layer, thereby reducing the dislocation and, in order to further reducing the dislocation density, a self-independent GaN substrate has been used. Use of a GaN substrate is advantageous in that a resonator end face in an LD can be formed into a cleaved surface and is very effective.
On the other hand, in a light-emitting device emitting light in the ultraviolet or deep-ultraviolet region and having a light-emitting layer of gallium nitride or aluminum gallium nitride (AlxGa1-xN (0<x≦1)), since the GaN absorbs light having a wavelength of 360 nm or less, the light emitted from the light-emitting layer is absorbed to lower the luminous efficiency. In the structure of a layer of AlxGa1-xN (0<x≦1) on GaN, cracks are easy to form due to the differences in lattice constant and in thermal expansion coefficient to hinder the fabrication of a device. The formation of cracks becomes conspicuous with an increase of an Al composition, and the larger the Al composition, the larger the affection thereof on devices of a short wavelength.
As a method to solve the problem, attempts have been made to form, in a region having a relatively small Al composition of 50% or less, an under layer of aluminum gallium nitride (AlxGa1-xN (0<x≦1)) via a GaN or AlN buffer layer on an under layer. In this method, however, it is difficult to suppress crack formation in a region having a large Al composition and, even when the crack formation can be suppressed, the dislocation density is large. Thus, such a substrate cannot be used as the substrate for a light-emitting device.
As a substrate having a large Al composition, a lettering guide substrate having AlN stacked on a base material of sapphire has been developed (see, for example, JP-A 2002-274996). In the case of the lettering guide substrate, the sapphire base material is deteriorated at a high temperature of 1500° C. or more and, at a higher temperature, it is impossible to stack aluminum gallium nitride (AlxGa1-xN (0<x≦1)). In view of the above and in the same manner as in the fact that a self-independent GaN substrate is effective for a blue LED with respect to an aluminum gallium nitride (AlxGa1-xN (0<x≦1)) Group III nitride semiconductor of ultraviolet or deep-ultraviolet region having a large Al composition, it is required that a self-independent substrate of AlN or AlxGa1-xN (0<x≦1) is used for the purpose of enhancing the crystal quality resulting from reduction in light absorption and dislocation density and also enhancing the cleaved surface property for the formation of an LD end face.
As regards the fabrication of an aluminum nitride substrate, while applying the GaN substrate fabrication technique, the sublimation method (see, for example, JP-A 2004-284870), the flux method (see, for example, JP-A 2004-231467), the HVPE method (see, for example, JP-A 2001-181097), the amonothermal process (see, for example, JP-A 2004-002152 and the solution technique (see, for example, App. Phys. Soc. Autumn General Meeting, 2004, papers, 1a-W-4) are attempted.
For the fabrication of a substrate of AlxGa1-xN (0<x≦1) that is a mixed crystal of AlN and GaN, a vapor phase growth method is dominant. The HVPE method is a vapor phase growth method and enables the compositions of AlN and GaN to be controlled and, therefore, has a high probability.
On the other hand, the MOCVD method that is a vapor phase growth method similar to the HVPE method has a feature capable of controlling the composition of a mixed crystal of AlN and GaN with higher precision than the HVPE method and is optimum as a method for depositing a layer of AlxGa1-xN (0<x≦1). The MOCVD method makes it possible to enlarge a device and is excellent in productivity. Almost all the LEDs, LDs and electronic devices using Group III nitride semiconductors, available on the market at present utilize the MOCVD method to grow crystals, from which it is found that the MOCVD method is an effective method. However, since the growth speed in the MOCVD method is generally several μm/hr that is lower than that of the HVPE method that is several tens of μm/hr, the MOCVD method is disadvantage for depositing a crystal in a large thickness. Therefore, it is still difficult to deposit a layer of AlxGa1-xN (0<x≦1) in a thickness of several tens to several hundreds of μm required as the thickness of a substrate.
If it is possible to deposit the crystal in a large thickness, with the growth speed increased in the MOCVD method, however, a substrate of AlxGa1-xN (0<x≦1) can be fabricated, with the composition of the AlxGa1-xN (0<x≦1) controlled with high precision.
An object of the present invention is to obtain a self-independent substrate of AlxGa1-xN (0<x≦1) through a method comprising using the MOCVD method to deposit a layer of AlxGa1-xN (0<x≦1) in a thickness of several tens to several hundreds of μm at high growth speed on a base material and then removing the base material, thereby materializing a Group III nitride semiconductor excellent in luminous efficiency in the ultraviolet and deep-ultraviolet regions. By using the substrate, it is made possible to provide a Group III nitride semiconductor light-emitting device excellent in luminous efficiency in the ultraviolet and deep-ultraviolet regions.